Self-Cleaning Materials and Surfaces

A Nanotechnology Approach

Walid A. Daoud, Walid A. Daoud

• Publisher: Wiley
• Year: 2013
• ISBN:
  9781118652367
• Edition: 1
• Topic: Tecnología e ingeniería
• Subtopic: Ciencias de los materiales

With increasing demand for hygienic, self-disinfecting and contamination free surfaces, interest in developing self-cleaning protective materials and surfaces has grown rapidly in recent times. This new title comprises of invited chapters from renowned researchers in the area of self-cleaning nano-coatings and the result is a comprehensive review of current research on both hydrophobic and hydrophilic (photocatalytic effect) self-cleaning materials.

Part I

Concepts of Self-Cleaning Surfaces

1

Superhydrophobicity and Self-Cleaning

Paul Roach1 and Neil Shirtcliffe2

1 Institute for Science and Technology in Medicine, Guy Hilton Research Centre, Keele University, UK

2 Faculty of Technology and Bionics, Hochschule Rhein-Waal, Germany

One of the ways that surfaces can be self-cleaning is by repelling water so effectively that water-borne contaminants cannot attach – by being superhydrophobic. This is demonstrated particularly well by the Indian Lotus, Nelumbo nucifera, which has leaves that remain clean in muddy water. The leaves can be cleaned of most things by drops of water, an effect that has been patented and used in technical systems [1].

1.1 Superhydrophobicity

1.1.1 Introducing Superhydrophobicity

Superhydrophobicity is where a surface repels water more effectively than any flat surface, including one of PTFE (Teflon®). This is possible if the surface of a hydrophobic solid is roughened; the liquid/solid interfacial area is increased and the surface energy cost increases. If the roughness is made very large, water drops bounce off the surface and it can become self-cleaning when it is periodically wetted. To understand more about this type of self-cleaning it is necessary to consider how normal surfaces become wetted and become dirty. The effect has been a focus of much recent research and has been reviewed recently [2–7]. A good mathematical explanation can be found in a recent book chapter by Extrand [8].

1.1.2 Contact Angles and Wetting

When a liquid rests on a surface the “contact angle” is measured through the droplet between the solid/liquid and liquid/air interfaces. The equilibrium angle that forms is known as Young's angle after a theory proposed by Young, but not actually formulated in his work [9]. Young's equation can be considered as a force balance of lateral forces on a contact line. In a perfect system the contact line cannot sustain any lateral force, so will always move to a position where the forces balance. This is achieved mathematically by taking the components of each force in the plane of the surface, at right angles to the contact line, as shown in Figure 1.1.

(1.1)

where

is the interfacial tension and the subscripts refer to solid, liquid and gas, for example,

is the interfacial tension between solid and liquid.

Figure 1.1 Cross-section of a drop on a flat surface with the contact angle θ. Contact angles also form at the edge of larger pools of water, in tubes, at bubbles on underwater surfaces and any other configuration where a liquid interface meets a solid.

Young's equation can also be derived from the surface and interfacial energies and their changes. The contact angle is an important measure of the interaction between the three phases, one solid, a liquid and another fluid, which may be a liquid or a gas. For small drops on a flat surface the drops form spherical caps, spheres intersecting the surface. External factors, such as electric fields, may also influence the drop shape, with gravity playing a role in distorting larger droplets. At the contact line the angle tends to the Young angle except when the contact line is moving relatively rapidly. In most systems there is a certain uncertainty in contact angle known as contact angle hysteresis.

1.1.3 Contact Angle Hysteresis

In practice the equilibrium angle is often difficult to measure because there are a small range of angles on every surface that are stable. These are often described as local energy minima close to the global energy minimum. In practice the contact line therefore often behaves as though it were fixed over a small range of angles close to the equilibrium angle [10]. Traditionally, the equilibrium contact angle was approached by vibrating the surface to supply the energy for the drop to escape the local minima. Although the static angle can vary, the contact line begins to move at a certain angle when the liquid front is advanced and at a different angle when it recedes. These values are simpler to measure so it is often the greatest stable angle and the lowest stable angle that are measured, known as the advancing and receding angles. The angles commonly quoted are those measured at a very low speed as the measured angles are affected by the speed of motion of the contact line. This is usually carried out by injecting liquid slowly into a drop and removing it again. Often the advancing and receding angles are of more practical use than the equilibrium angle, although the equilibrium value can be used to derive surface energies. It is sometimes possible to determine the equilibrium angle if both advancing and receding angles are measured. This still assumes that hysteresis is not very large and the surface is reasonably flat [11].

The difference between the advancing and receding angles, or rather the difference between the cosines of the angles governs whether liquids will stick to a surface or slide or fall off. A drop on a vertical sheet can have the advancing angle at the bottom and the receding angle at the top without moving (Figure 1.2). Surfaces with low hysteresis allow drops to slide over them whatever the equilibrium contact angle. The energy required for a drop to move can be calculated as [12],

(1.2)

where r is the base radius of the drop. The contact angle itself enters the equation in two ways: first the cosine function enhances differences near 90°; secondly the value of the contact radius r, for a given volume depends upon the contact angle.

Figure 1.2 A drop on a vertical surface sliding slowly with advancing angle at the front and receding angle at the back, in practice geometrical factors and speed of movement will change the angles away from the actual advancing and receding angles.

Furmidge calculated the angle of tilt, α, required in order for a drop to slide [13],

(1.3)

where w is the width of the drop.

Measurement of the force required to remove drops from surfaces and tilting angles shows the general trend is correct but some differences can be measured, particularly for softer surfaces. Going back to Young's equation, if the force balance approach is used, the surface tension components in the plane of the solid are balanced to give the contact angle, but this leaves a vertical force on the surface, depending upon contact angle. Theories by de Gennes and Shanahan [14] and experiments on soft materials suggest that this force distorts the surface, generating a ring like an atoll around the base of the drop and increasing the force restraining the drop from sliding on the surface. Of course the drop profile is also far from a circle if hysteresis is significant, particularly for large drops (for example that shown in Figure 1.2).

The receding angle (and liquid properties) controls whether a drop falls off an inverted surface, the advancing angle is not involved as it is never reached in this case.

The work needed to pull a liquid from a surface has been reported to be determined by [15,16].

(1.4)

1.1.4 The Effect of Roughness on Contact Angles

1.1.4.1 Fully Wet Surfaces; Wenzel's Equation

As the roughness is increased the water initially wets the entire surface, as shown in Figure 1.3b, the increasing surface area of the interface means that the advancing contact angle on a surface with a flat contact angle of greater than 90° increases, whereas that of one below 90° decreases. A surface with exactly 90° contact angle would show no effect of roughness. This type of wetting can, therefore, be considered to be an amplification of the properties of the surface by the roughness. The contact angle of a rough surface of this type can be calculated using Wenzel's equation [17], which modifies the cosine of the angle by the specific surface area, r, the amount of times the surface is larger than a flat surface of the same size. The subscript e has been used to highlight that usually the equilibrium contact angle is considered as opposed to the receding or advancing contact angles introduced in Section 1.1.3.

(1.5)

Figure 1.3 Wetting on flat and rough surfaces: (a) flat, (b) rough, Wenzel case; (c) Cassie and Baxter case.

The amplification of both hydrophilicity and hydrophobicity arises from the change in sign of cosθ at 90°.

1.1.4.2 Bridging the Roughness; Cassie and Baxter's Equation

If the surface is roughened it eventually becomes energetically favourable for the liquid to sit on the top of the roughness and reduce the area of the interface, as shown in Figure 1.3c. In this case the state approaches that of a liquid on a flat surface with domains of different contact angles but where one of the materials is the second fluid (in this example air).

The simplest expression for the contact ...

Table of contents

• Cover
• Title Page
• Copyright
• List of Contributors
• Preface
• Part I: Concepts of Self-Cleaning Surfaces
• Part II: Applications of Self-Cleaning Surfaces
• Part III: Advances in Self-Cleaning Surfaces
• Part IV: Potential Hazards and Limitations of Self-Cleaning Surfaces
• Index